Area Forecast Discussion
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FXUS61 KPHI 231512
Area Forecast Discussion
National Weather Service Mount Holly NJ
1112 AM EDT SAT JUL 23 2016
Frontal boundary sinks through the region today. Then high
pressure builds over the region through the weekend. By Monday, a
cold front will approach from the west, but may stall over the
region through mid week. High pressure may build in from the north
for next weekend.
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Mid morning surface analysis shows a surface low over the Canadian
Maritimes. A cold front extended to the west-southwest along the
Northeast U.S.-Eastern Canada border. Meanwhile, a pre-frontal
trough in the Gulf of Maine extended south-southwestward into the
mid-Atlantic region. Over our area, the surface trough was
positioned several miles to the south/east of the I-95 but oriented
parallel to the major interstate. Although not quite noticeable
right now, a contrast in dewpoints on either side of this boundary
will become evident this afternoon as the surface trough sharpens
and drier air aloft mixes down to the surface. Dewpoints that are
currently in the upper 60s and low 70s with a west to northwest wind
behind (i.e., to the north and west) the boundary are expected to
fall into the 50s and low 60s later today. Farther south into the
Delmarva and far southern NJ, conditions will remain humid with
dewpoints steady in the upper 60s and low 70s.
The Excessive Heat Warning will go into affect late this morning for
the urban I-95 centers including Wilmington, Philadelphia and
Trenton metro where heat indices are forecast to be in the 97-103F
range this afternoon. We will also keep the Heat Advisory in place
farther east into central/ southern NJ where heat indices are
forecast to be in the 98-104F range and farther south into eastern
MD and DE where it will be much more humid and thus heat indices are
forecast peak in the 103-109F range. Farther north across
northeastern PA and northwestern NJ, heat indices away from of the
cooler mountains will be in the 90-95 range with much drier air in
Hourly PoPs and Weather grids were updated through tonight with the
mid morning ESTF update. The hi-res convective-allowing model
guidance are all in pretty good agreement showing convection
initiating just south of the front near the Mason-Dixon line and
southern NJ by about 3 PM. Showers and thunderstorm development
expanding southward into the Delmarva late this afternoon. Coverage
of showers and storms should mainly be isolated. Better coverage is
expected this evening across our southern zones with additional lift
arriving with a mid-level shortwave trough.
.SHORT TERM /6 PM THIS EVENING THROUGH 6 AM SUNDAY/...
As the front continues to sink southward through the area and the
short wave/vorticity impulse continues southward as well, showers
and thunderstorms could continue into the evening across the
southern portions of the area. So we will keep the slight
chance/chance POPS across the southern half of the area through the
evening hours. Once the front and short wave move south of the area
around or shortly after midnight, any precipitation will come to an
With lowering dewpoints and lightening winds, some areas could be
several degrees cooler tonight compared to Friday night.
.LONG TERM /SUNDAY THROUGH FRIDAY/...
Sunday...Surface high entrenched north of our region along with
and upper level short wave ridge crossing the NE US at this time
should keep our region dry on Sunday. With weak surface flow,
expect another day of very low relative humidity values. However,
unlike today, tomorrow winds should be light.
Monday...low level southwesterly flow ahead of the next cold
front develops. This, combined with increasing 1000-500mb
thicknesses thanks to the previously mentioned short wave ridge
will lead to the hottest day of the next week, and possibly the
hottest day we`ve seen so far this year. Additionally, the
southwesterly flow will also be responsible for moisture
advection, leading to increasing dew points and heat index values
above 100 for most of the area. As the front, and an upper level
short wave trough, approaches late in the day, could see scattered
thunderstorms develop. Coverage may be limited thanks to limited
shear and warm mid levels, but if any storms develop, they could
have downburst potential with gusty winds. As a result, there is a
marginal risk for severe storms for some of our region. Will hold
off on mentioning in the HWO for now.
Tuesday and Wednesday...relief behind the cold front will be more
in the form of lower relative humidity values, while temperature
wise, we should only be a few degrees lower as compared to Monday.
depending on where the front stalls, we could see showers and
thunderstorms develop along the stalled front each afternoon and
Thursday and Friday...as the cooler air mass continues to slide
south, a modest cooling trend should continue. In the upper
levels, expect mostly a zonal flow pattern. As such, could see a
typical summertime convection pattern with isolated to scattered
showers and thunderstorms each afternoon and evening.
.AVIATION /15Z SATURDAY THROUGH WEDNESDAY/...
The following discussion is for KPHL, KPNE, KTTN, KABE, KRDG, KILG,
KMIV, KACY and surrounding areas.
VFR conditions are expected across the area today and tonight for
most locations. There is a chance of isolated/scattered showers or
thunderstorms across the southern half of the area today into this
evening. However, being they are forecast to be isolated/scattered
in nature, we have left them out of the TAFS for now, although we
may need to include the in a TEMPO group in the future. If a shower
or thunderstorm did develop, they could briefly lower VSBYS.
Some patchy fog may develop across mainly the southern portions of
the area later tonight, especially if any precipitation falls across
Southwest-west winds will become more west-northwest through the
morning and into the afternoon as a frontal boundary moves across
the area. Winds are expected to become gusty around 15-20 knots
through today as well. The gusts will drop off this evening, and
eventually winds will become light and variable most places
overnight. Any directions overnight would be more northwest, become
northeast by daybreak.
Sunday...VFR conditions expected
Monday...MVFR or even brief IFR conditions possible with scattered
thunderstorms...primarily in the afternoon and evening.
Tuesday and Wednesday...mostly VFR conditions expected. A small
chance of thunderstorms on the Coastal Plains and southern
Delaware Valley (including KACY...KMIV...KILG...and KPHL) on
W-NW winds are currently 10-15 kt in our NJ coastal waters from
ANZ452 northward and under 10 kt farther south in our coastal
waters and DE Bay. We expect the winds to back out of the SW or
even S-SE in spots this afternoon once the sea-breeze develops a
few miles offshore. S-SWly winds are forecast to be strongest late
this afternoon and this evening in our coastal waters off S NJ and
DE. Winds could be sustained near 20 kt but the potential to mix
down higher gusts is minimal with a hot airmass residing over the
relatively cooler waters. While there could be a few gusts near
25 kt late in the day nearshore, do not expect conditions will
warrant a SCA. Medium-period sly swells will persist today. Wave
heights should subside to 2-3 ft this afternoon after starting
3-4 ft this morning.
Sub-Small Craft Advisory conditions are expected today and tonight.
However, winds could gust around 20 knots today. A frontal boundary
will sink southward across the waters later today and into the
overnight hours. It is possible that isolated/scattered showers or
thunderstorms may affect the waters later today into this evening.
Sunday...winds and seas should remain below SCA criteria.
Monday...winds and seas on the coastal waters may approach or even
reach SCA criteria primarily from the afternoon through Monday
Tuesday through Wednesday...winds and seas should remain below SCA
Rip Currents...Offshore winds today will make it unfavorable for
the sea-breeze front to progress inland this afternoon, except may
at the barrier islands. The W-NW winds that are currently
occurring this morning should back more out of the SW or even S
this afternoon and increase ahead of a pressure trough from about
LBI southward. This would enhance the potential for the formation
of dangerous rip currents at the south-facing beaches in Cape
May. After coordinating with local beach patrol on our daily surf
conference call, we opted to go with a moderate risk for rip
currents in Cape May County. Lifeguards at Cape May had rip
current issues yesterday and expect the same today with the setup
being similar to yesterday. Elsewhere, kept the risk low, but it
will likely be at the upper end of it later this afternoon as
winds increase and become oriented parallel to the shore.
With rising temperatures and lowering dewpoints, relative humidity
values are forecast to drop near 30 percent later today across
the northern two-thirds of NJ and eastern PA. Meanwhile, west-
northwest winds will increase to 10-15 mph and could gust in the
low 20s. Summertime is not usually a period when we are concerned
about fire weather, but contacted our fire weather partners in NJ
and PA to determine how dry the fuels are and if any enhanced
statements should be issued. A statement does not appear necessary
at this time unless the wind speeds become much higher than
Farther south across Delaware and Maryland, RH values are not
expected to drop below 35-40 percent, so we have less concerns
that far south.
Record high temperatures today through Thursday are below.
Site 23rd 24th 25th 26th 27th 28th
---- ---- ---- ---- ---- ---- ----
PHL... 101-2011 98-2011 96-1899 101-1892 101-1940 100-1941
ABE... 99-1955 95-1999 95-1999 98-1940 98-1955 97-1949
ACY... 105-2011 100-2011 99-2010 96-2011 99-2005 98-1999
ILG... 100-2011 98-2011 96-1987 99-1894 100-1894 101-1894
TTN... 104-2011 98-1910 97-1999 99-1892 100-1894 101-1894
GED... 104-2011 99-2011 99-2010 97-2012 98-2005 99-1949
RDG... 100-2011 96-2010 96-1999 99-1940 98-1955 99-1941
MPO... 91-1955 91-1914 90-1999 89-1949 91-1955 93-1949
PA...Excessive Heat Watch from Monday morning through Monday
afternoon for PAZ060>062-101-103-105.
Excessive Heat Warning until 6 PM EDT Monday for PAZ070-071-
NJ...Excessive Heat Watch from Monday morning through Monday
afternoon for NJZ007>010-012>014-016-020>027.
Heat Advisory until 8 PM EDT this evening for NJZ012>014-016-
Excessive Heat Warning until 6 PM EDT Monday for NJZ015-
DE...Excessive Heat Watch from Monday morning through Monday
afternoon for DEZ002>004.
Heat Advisory until 8 PM EDT this evening for DEZ002>004.
Excessive Heat Warning until 6 PM EDT Monday for DEZ001.
MD...Excessive Heat Watch from Monday morning through Monday
afternoon for MDZ008-012-015-019-020.
Heat Advisory until 8 PM EDT this evening for MDZ008-012-015-